Inorganic colorants contain various types of metal ions in the form of carbonates, sulfides and oxides. The nature of the metal ion plays a vital role in the color of the colorant. Several colorants conventionally used in the paint industry are toxic. These include barium chromate, cadmium sulfide, lead antimoniate etc. Regulations in Europe concerning the protection of the environment have forced the paint industry to invest heavily in research programs on development of inorganic paints that are free of heavy metals such as lead, mercury, cadmium, antimony, arsenic, chromium, selenium etc. Rare earth elements offer a vast opportunity for development of environmentally secure alternatives for many of the eco-constrained colorants. Their unique electronic configuration of partially filled f orbitals result in unusual magnetic and optical properties. The color developed depends on the number of unpaired electrons. The rare earth elements derive their color from charge transfer electronic transitions. As reported by Maestro et al (Journal of Alloys and Compounds, Vol. 225, p. 520, 1995), the preparation of colorants based on rare earth elements has been based on the use of individual rare elements as their oxides, sulfides and phosphates. As reported by Lee (Concise Inorganic Chemistry, 4th Edition, ELBS publication, p. 864, 1991), the separation of one rare earth element from another is an exceedingly difficult task, almost as difficult as the separation of isotopes of one element. This is due to the similarities in size and charge of the rare earth elements. This results in high cost of obtaining individual rare earth elements, and hence only a fewer rare earth based colorants are industrially available, with most of them as phosphors and luminescent colorants. As reported by Pan et al (Journal of Solid State Chemistry, Vol. 174, p. 69, 2003), Lanthanide-doped nanocrystalline oxides with particle diameters of 100 nm or less have been drawing particular interest as phosphors. Ravilsetty P Rao (U.S. Pat. No. 5,989,454, 1999) has described a method for making small particle blue emitting lanthanum phosphate based phosphors. The importance of donor- and acceptor-like charge transfer processes in the luminescence properties of rare earth doped crystalline solids have become increasingly evident. As reported by Yen et al (Journal of Luminescence, Vol. 69, p. 287, 1996), small, but crucial, differences in the relative position of the orbitals can result either in complete quenching of the emission or in luminescence with quantum efficiencies close to unity. The favorable influence of the use of oxides of rare earths for the preparation of red, blue and green phosphors is now known. In these preparations a relatively high level of purity of the rare earth compounds remains crucial. Mixed metal compounds with exact ratios of binary or tertiary systems are known to provide red (Y2-xEuxO3) or green (La1-x-yCexTbyPO4) phosphors, where x and y are integers.
Some rare earth oxides are being widely used in ceramic industry to produce colors. Sulcova et al (Dyes and Pigments, vol. 40, p87, 1998 and Dyes and Pigments, vol. 47, p285, 2000) have employed cerium oxide to develop an opaque white color and by doping cerium with praseodymium, neodymium, yttrium the other different shades like yellow, violet, orange and burgundy have been obtained.
Sulcova (Dyes and Pigments, vol. 47, p285, 2000) has reported that colorants of the formula Ce0.95-yPr0.05NdyO2-0.5y prepared by high temperature calcinations of cerium, praseodymium and neodymium oxides have a reddish hue, where y=0.05, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, 0.85. Gonzalvo et al (Journal of Alloy Compounds., vol. 323-324, p. 372, 2001) have reported that colored oxides of the type Bi2O3—R2O3 can serve as ecological inorganic colorants. Doping of alkali or alkaline earth metals with cerium results in good colorants. Swiler et al (U.S. Pat. No. 6,582,814, 2003) have reported that rare earth-transition metal oxide, preferably of the formula (RexTm)Oy, where Re is at least one rare earth element, Tm is at least one transition metal, x ranges from 0.08 to 12, and y ranges from x+1 to 2x+2 are useful as colorants, and possess good stability. Jansen et al (Nature, vol. 404, p. 980, 2000) have reported that calcium, lanthanum, tantalum oxynitrides provide for colors ranging from yellow to red.
In these colorants, the ratio of R (the rare earth ion) becomes increasingly relevant with the color being dependent on the type of rare earth oxide used. However, because of the close similarities in the chemical properties of the rare earth ions, separations are difficult and lead to higher costs of purified rare earth based compounds.
There is no prior information available on the use of mixed rare earth compounds (without separation as individual rare earth compounds) by suitable combination with other metal ions. Such an approach gains significance.